The present invention relates to a telecommunication network for establishing radiofrequency links between gateways and ground terminals via a multispot (called also multispot beam) telecommunication satellite. This type of satellite enables the use of several spot beams from antennas on board the satellite to cover contiguous geographic areas or cells, instead of a single large spot beam.
Such multispot satellites enable several radiofrequency links occupying the same frequency band on different spot beams to be established.
In the case of a broadband satellite telecommunication system, the satellite is used bidirectionally, which is to:                relay data sent by a gateway (connected to the ground network) to a plurality of ground terminals: this first point to multipoint type link constitutes the forward link;        relay to the gateway data sent by the ground terminals: this second multipoint to point type link constitutes the return link.        
An example of a forward link in a multispot telecommunication network is illustrated in FIG. 1.
Signals are sent to a multispot satellite 3 over an uplink LM by a gateway 2 (also called a central station) such as a gateway connected to an Internet backbone 5. The gateway controls the network through a network management system that allows the operator to monitor and control all the components in the network. The signals sent by the gateway are then processed at the level of satellite 3 that amplifies the signals, derives the signals at a generally lower frequency and then retransmits the signals from the satellite antenna or antennas on a downlink LD in the form of a plurality of spot beams or spots forming basic coverage areas or cells C1 to C8 in which ground terminals 6 are situated. Each cell C1 to C8 is associated with a spot beam SP1 to SP8. It will be noted that, in the case of configuration 1, the eight cells C1 to C8 respectively associated with eight spot beams SP1 to SP8 form a group of cells served by the same gateway 2. The return link from the ground terminals 6 to the gateway 2 operates identically with an opposite communication direction.
Coordination of frequencies between operators is done in the context of regulation issued by the International Telecommunication Union (ITU): thus, by way of example, the band Ka for region 1 (Europe, Africa, Middle East) is defined in table 1 below:
TABLE 1Forward linkUplink (from the gateway)27.5 GHz to 29.5 GHzDownlink (to the ground19.7 GHz to 20.2 GHzterminals)Return linkUplink (from the ground29.5 GHz to 30.0 GHzterminals)Downlink (to the gateway)17.7 GHz to 19.7 GHz
It is observed that the spectrums from band Ka in uplink are adjacent (i.e., the intervals [27.5; 29.5] and [29.5; 30] do not present any discontinuity). The same is true for spectrums from band Ka in downlink (i.e., the intervals [17.7; 19.7] and [19.7; 20.2] do not present any discontinuity).
Given that the gain from an antenna is inversely proportional to the opening of the spot beam, using multispot antennas to cover an extended zone with a homogeneous and elevated gain is necessary. The larger the number of spot beams, the smaller the opening of each spot beam will be. Thus, the gain on each spot beam and so the gain on the service area to cover will be increased. As we mentioned above, a service area to cover is formed by a plurality of contiguous cells (basic coverage areas), one spot beam being associated with each cell. A homogeneous multispot coverage area SA is represented in FIG. 2a), each cell being represented by a hexagon FH such that the coverage area is comprised of a plurality of hexagons FH in which θcell is the outer size of the cell expressed by the angle of the satellite associated with the coverage. However, as the antenna spot beam associated with each cell is not capable of producing a hexagonal form, a good approximation consists of considering a plurality of circular spot beams FC such as represented in FIG. 2b). The association of a spot beam with a cell is done by considering the best performance of the satellite for said spot beam, particularly in terms of EIRP (Equivalent Isotropically Radiated Power) and G/T figure of merit (gain to noise temperature ratio): a cell is determined to be the part of the service area associated with the spot beam that offers the highest gain on this zone from among all the satellite spot beams.
Configuration 1 such as represented in FIG. 1 uses a technique known as the frequency reuse technique: this technique enables the use of the same frequency range several times in the same satellite system in order to increase the total capacity of the system without increasing the allocated bandwidth.
Frequency reuse schemes, known as color schemes, making one color correspond to each of the satellite spot beams, are known. These color schemes are used to describe the allocation of a plurality of frequency bands to the satellite spot beams in view of radiofrequency transmissions to carry out in each of these spot beams. In these schemes, each color corresponds to one of these frequency bands.
In addition, these multispot satellites enable the sending (and receiving) of polarized transmissions: the polarization may be linear (in this case the two polarization directions are horizontal and vertical, respectively) or circular (in this case the two polarization directions are left circular or right circular, respectively). It will be noted that in the example from FIG. 1, the uplink leaving the gateway 2 uses two polarizations with four channels for each polarization, respectively Ch1 to Ch4 for the first polarization and Ch5 to Ch8 for the second polarization: the use of two polarizations allows the total number of gateways to be reduced. The eight channels Ch1 to Ch8, after processing by the payload of the satellite 3, will form the eight spot beams SP1 to SP8 (one channel being associated with one spot beam in this example).
According to a four-color scheme (red, yellow, blue, green) with a frequency spectrum of 500 MHz for each polarization, the transmissions being polarized in one of two right circular or left circular polarization directions, each color is associated with a 250 MHz band and a polarization direction.
In the rest of the description, we will take the following convention:                the color red is represented by lines hatched to the right;        the color yellow is represented by dense dots;        the color blue is represented by lines hatched to the left;        the color green is represented by dispersed dots.        
A color is thus associated with each satellite spot beam (and thus a cell) such that the spot beams with the same “color” are non-adjacent: contiguous cells thus correspond to different colors.
An example of a four-color scheme for covering Europe is represented in FIG. 3. In this case, 80 cells are necessary to cover Europe.
This type of scheme is applicable equally well in uplink and in downlink. At the satellite level, a spot beam is created from a horn radiating towards a reflector. A reflector may be associated with a color such that four-color coverage is ensured by four reflectors. In other words, the generation of 16 spot beams from each gateway may be done by using four antennas (one per color) each having a reflector, four horns being associated with each reflector.
FIG. 4 illustrates a frequency plan broken down into an uplink frequency plan PMVA on the forward link, a downlink frequency plan PDVA on the forward link, an uplink frequency plan PMVR on the return link and a downlink frequency plan PDVR on the return link. The notations RHC and LHC respectively designate the right and left circular directions of polarization.
The PMVA plan corresponding to the forward uplink (from the gateway to the satellite) disposes 2 GHz (from 27.5 to 29.5 GHz) of available frequency spectrum such that 16 channels of 250 MHz of bandwidth are generated by a gateway (8 channels for each polarization). These 16 channels, after processing by the satellite payload, will form 16 spot beams. The assumption made here consists of considering that the entire 2 GHz spectrum is used: however, it will be noted that it is also possible, particularly for operational reasons, to use only one part of the spectrum and to generate fewer channels. In the example above, 16 spot beams (and thus 16 cells) are generated from two signals multiplexing the 8 channels (a signal multiplexed by polarization) generated by a gateway. Each multiplexed signal corresponding to a polarization is then processed at the satellite transponder level so as to provide 8 spot beams; each of these spot beams is associated with a frequency interval among the two frequency intervals [19.7; 19.95] and [19.95; 20.2] and with an RHC or LHC polarization as represented on the downlink frequency plan PDVA.
The PDVR plan corresponding to the return downlink (from the satellite to the gateway) disposes 2 GHz (from 17.7 to 19.7 GHz) of available frequency spectrum such that 16 spot beams of 250 MHz of bandwidth (associated with a frequency interval from among the two frequency intervals [29.5; 29.75] and [29.75; 30] and with an RHC or LHC polarization such as represented on the downlink frequency plan PMVR) issued from cells are multiplexed at the satellite level into two signals (corresponding to each polarization) to be returned to the gateway (8 channels for each polarization). We are still assuming that the entire 2 GHz spectrum is used.
It will be noted that the four-color scheme, for the forward link, associates one of the four following colors with each spot beam belonging to a pattern of four adjacent spot beams:                a red color corresponding to a first band of 250 MHz (lower part of the available spectrum of 500 MHz) and to the right circular polarization direction;        a yellow color corresponding to the same first band of 250 MHz and to the left circular polarization direction;        a blue color corresponding to a second band of 250 MHz (upper part of the available spectrum of 500 MHz) and to the right circular polarization direction;        a green color corresponding to the same second band of 250 MHz and to the left circular polarization direction;        
The four adjacent spot beams with the same pattern are each associated with a different color.
On the return link, the polarizations are reversed so that the red and yellow colors have a left circular polarization and the blue and green colors have a right circular polarization. The ground terminals send and receive according to an opposite polarization such that one may easily separate the uplink signals from the downlink signals: such a configuration enables less costly terminals to be used.
In the case where the gateways are situated in the service area, it should be noted that the gateways are located in cells and thus share the same horn with the users. Subsequently, these particular cells will be designated by the term “gateway cell.”
Thus, in uplink, the signal sent by each gateway cell is demultiplexed by a demultiplexer at the satellite level so as to separate the gateway signal from the ground terminals signal.
In addition, in downlink, the signal sent by the satellite to each gateway cell is multiplexed by a multiplexer at the satellite level so as to mix the signal intended for the gateway and the signal intended for the ground terminals.